The present invention relates to a plaster mixture for forming a machinable composition, and specifically to a plaster mixture which can be adapted to be poured into a mold, trowel applied, or spray applied to a substrate, and machined to a desired shape to produce a dimensionally accurate model.
In some types of manufacturing, the molds used in production are cast from models. One method of making a model involves shaping a substrate with CNC or other high performance milling machine. In applications where the models are large, such as in the automobile, watercraft and aerospace industries, a large volume of substrate material is necessary to build a model. Therefore, it is desirable that the substrate material be lightweight and economical. Also, the material must be machinable to a high degree of dimensional accuracy. Accordingly, the material must be able to hold a sharp corner and a smooth contour and be dimensionally stable from room temperature through 250xc2x0 F.
To produce large models, it is common to first fabricate a substrate that is sized slightly less than the full size model. Inexpensive materials such as tooling boards, Styrofoam and the like are commonly used to fabricate the substrate. The substrate is coated with successive layers of a machinable material until the substrate is larger than the model. Finally, the coated substrate is machined into the desired shape.
Plaster mixtures for forming machinable compositions are known. Notably, U.S. Pat. No. 5,534,059 to Immordino describes a high density gypsum composition for use as a machinable composition, which is readily carvable, is able to hold sharp corners and smooth contours, and is dimensionally stable from room temperature to 250xc2x0 F. This plaster contains a polymer binder for increasing strength of the plaster and for preventing the generation of dust during machining. As disclosed in the Immordino patent, the binder is an ethylene/vinyl/acetate/vinyl chloride terpolymer. Up to 10 to 20 percent by weight of polymer binder is preferably added to the plaster to obtain the desired consistently. The composition of Immordino discloses the addition of an internal lubricating agent to minimize friction during machining. Further, this composition may not be spray applied to a substrate since, if sprayed, due to the effects of gravity, the composition will flow prior to set and before building up a machinable thickness.
Machinable materials adapted to be spray applied on a substrate are also known, namely, polyester and polyurethane foams. Polyester is undesirable because it is possible to apply only about a {fraction (1/16)} inch coat at a time. Therefore, numerous coats of polyester are needed to build tip an appreciable thickness. Also, because each coat must be allowed to dry before the next coat, this method is time consuming.
Polyurethane foams are undesirable because they do not produce a coat with a uniform thickness and the coats normally have numerous air pockets. Therefore, to achieve dimensional accuracy, significant surface preparation is necessary following machining. Polyester and polyurethane foams generate static electricity and undesirable amounts of dust during machining. The combination of the static and the dust creates an unwanted mess. Further, the isocyanate used for curing the polyurethanes is a known carcinogen.
Thus, it is a first object of the present invention to provide an improved machinable composition which can be found from a plaster mixture, poured into a mold or applied by trowel.
Another object of the present invention is to provide an improved plaster mixture which is both sprayable and forms a composition which is machinable.
Yet another object of the present invention is to provide an improved sprayable plaster mixture which will build up a machinable thickness in a single coat when sprayed on a substrate.
It is a further object of the present invention to provide an improved sprayable plaster which can form a composition which generates shavings and relatively little dust when machined.
Yet another object of the present invention is to provide an improved sprayable plaster that provides for relatively little expansion and a relatively small number of air pockets upon set.
Still another object of the present invention is to provide an improved method for making a model using a sprayable plaster mixture for forming a machinable composition.
The above listed objects are met or exceeded by the present sprayable plaster mixture which can be poured into a mold or applied by spray or trowel to a substrate and machined to a desired shape after set. The present invention also provides a method for using the plaster mixture to economically produce large models.
More specifically, the a plaster mixture is provided for forming a machinable composition, the mixture includes calcium sulfate hemihydrate, an internal lubricating dispersable binder; and an adhesive binder for adhering the plaster mixture to a substrate. In the preferred embodiment, the composition includes calcium sulfate hemihydrate, amorphous poly(ethylene glycol) internal binder, and an ethylene-vinyl acetate copolymer based adhesive binder.
A second embodiment featuring a sprayable slurry capable of adhering to a horizontal surface is achieved with the addition of a thickening agent to the first embodiment. The thickening agent allows the slurry to form a machinable thickness when spray applied. In the preferred second embodiment, the thickening agent is cellulose based, and is preferably methylcellulose.
A third embodiment featuring a slurry capable of adhering to both vertical and horizontal surfaces is achieved with the addition of an accelerating agent to the second embodiment. The accelerating agent reduces the slurry set time after the slurry is spray applied to a substrate.
A lightweight inert filler may be added to any of the three embodiments to reduce the machinable thickness final weight and also to adjust the slurry density. Further, any of the three embodiments may also include a defoaming agent, set accelerants and/or set retarders to suit particular applications.
In addition, the present invention features a method for making a model by spraying one or more coats of the mixture described in the third embodiment onto a substrate and allowing the slurry to set to form a machinable thickness. Once set, the substrate is machined to the desired shape of the model.